In the fabrication process for semiconductor devices, numerous fabrication steps, as many as several hundred, must be executed on a silicon wafer in order to complete integrated circuits on the wafer. These steps typically include sequential deposition of conductive and insulative layers on the silicon wafer substrate; formation of a photoresist or other mask such as titanium oxide or silicon oxide, in the form of the desired metal interconnection pattern, using standard lithographic or photolithographic techniques; subjecting the wafer substrate to a dry etching process to remove material from one or more conducting layers from the areas not covered by the mask, thereby etching the conducting layer or layers in the form of the masked pattern on the substrate; removing or stripping the mask layer from the substrate typically using reactive plasma and chlorine gas, thereby exposing the top surface of the conductive interconnect layer; and cooling and drying the wafer substrate by applying water and nitrogen gas to the wafer substrate.
The numerous processing steps outlined above are used to cumulatively apply multiple electrically conductive and insulative layers on the wafer and pattern the layers to form the circuits. Additional techniques, such as dual damascene processes, are used to form conductive vias which establish electrical contact between vertically-spaced conductive lines or layers in the circuits. Tungsten plugs are frequently formed in via openings to establish electrical communication between conductive layers separated by an insulating layer. The finished semiconductor product includes microelectronic devices including transistors, capacitors and resistors that form the integrated circuits on each of multiple die on a single wafer.
Since the processing of silicon wafers requires extreme cleanliness in the processing environment to minimize the presence of contaminating particles or films, the surface of the silicon wafer is frequently cleaned after each processing step. For instance, the wafer surface is cleaned after the deposition of a surface coating layer such as oxide or after the formation of a circuit by a processing step such as etching. A frequently-used method for cleaning the wafer surface is a wet scrubbing method.
In cleaning a wafer surface by a wet scrubbing method, a wafer is rotated at a high speed, i.e., at least about 200 RPM and preferably, about 1,000 RPM, simultaneously with a jet of high-pressure deionized water sprayed on top. The water jet is normally sprayed at a pressure of about 2,000–3,000 psi. The water movement on top of the wafer surface displaces any contaminating particles that are lodged on the wafer surface.
One limitation of the wafer jet scrubbing method is that the process only moves particles from side to side in openings, such as oxide windows, without removing the particle. Furthermore, as the image size decreases, it becomes more difficult for water to reach the particles in openings because of increased surface tension.
It has also been noted that in a water jet scrubbing process conducted on a silicon wafer that is coated with an insulating material, i.e., an oxide layer as an inter-metal dielectric layer, some regions of the film are damaged at the wafer center by the cumulated stress from the water jet when the aperture size of the jet nozzle is too large or is distorted. The damaged film can be identified by a KLA scan, even though a large number of wafers must be tested since the probability of such damage is only about 10–30%.
In a jet-scrubbing process, a wafer is normally positioned on a wafer platform which is typically rotatably mounted on a wafer stage. The wafer platform rotates the wafer at a predetermined rotational speed, which may be between typically about 200 RPM and about 2,000 RPM. Simultaneously, a water jet of de-ionized water is ejected onto the upper surface of the rotating wafer from a nozzle opening in the nozzle. The water jet has a water pressure of typically about 50 kg/cm2.
As it strikes the surface of the wafer at an angle of typically about 45°, the water jet is scanned along a top of the wafer surface by a lateral sweeping motion of the water jet nozzle to define a generally curved or arcuate trace which normally traverses the center of the wafer. The surface of the wafer is scanned by the water jet at least once, and preferably, several times. Centrifugal force acting on the water flow on the surface of the wafer due to the rotating wafer platform and wafer removes contaminating particles or films from the surface of the wafer. Horizontal movement of the wafer stage beneath the water jet nozzle during the scrubbing process provides a more uniform dispersement of the sprayed water along the entire surface of the disc. After completion of the jet-scrubbing process, the wafer is subjected to a spin-drying step in which the wafer is rotated and clean dry air (CDA) is blown against the wafer surface.
In the formation of tungsten plugs as metal interconnects on a wafer, an intermetal dielectric (IMD) layer is deposited on conductive metal lines previously etched from a metal layer on the wafer. Via openings are etched through the IMD layer, followed by the deposition of electrically-conductive metal via plugs, typically tungsten, in the via openings. A metal layer remains on the upper surface of the IMD layer after formation of the via plugs. This metal layer is removed by CMP prior to further processing. Upon completion of the CMP step, the upper surface IMD layer is exposed. The conventional process for cleaning the wafer after the CMP step involves subjecting the wafer to jet-scrubbing, followed by spin-drying. High-speed rotation of the wafer during the spin-drying step causes electrostatic charges to accumulate on the wafer. Consequently, electrically-charged particles adhere to the wafer and must be removed prior to further fabrication of the IC devices on the wafer.
In a jet-scrubbing process, the cleaning efficiency of the water jet ejected onto the wafer depends on various factors including the pressure and size of the water jet, as well as the scan density of the water jet on the wafer. It is well-known that ejecting a water jet of high scan density onto the surface of a wafer effectively removes particles from the wafer. However, in cases in which the jet-scrubbing process is used to clean via plugs after the plugs have been subjected to CMP, it has been found that jet-scrubbing of the wafer is inadequate to remove small particles and electrostatic charges from the exposed surfaces of the intermetal dielectric (IMD) layer and via plugs. This frequently results in the formation of a layer having a high electrical resistance on the exposed portions of the via plugs.
It has been found that subjecting via plugs on a wafer to cleaning by immersion or soaking the wafer in deionized (DI) water, followed by drying of the wafer using isopropyl alcohol (IPA), is effective in removing both particles and surface electrostatic charges from the wafer. This results in a via Rc (electrical resistance) reduction and yield improvement of greater than 1% in the finished IC devices. Accordingly, a method of improving IC device performance by cleaning via plugs on a wafer using DI water immersion or soaking, followed by IPA drying, is needed.
An object of the present invention is to provide a method of improving the performance and yield of IC devices on a semiconductor wafer.
Another object of the present invention is to provide a method of improving the performance and yield of IC devices on a wafer by thoroughly removing particles and electrostatic charges from the wafer after a CMP process.
Still another object of the present invention is to provide a method of improving the performance and yield of IC devices on a wafer which includes fabricating via plugs on the wafer, isolating the via plugs using CMP, soaking the wafer by immersing the wafer in deionized water, and subjecting the wafer to a drying process using isopropyl alcohol (IPA).